add text to multiple photos

No, it has to be one at a time. There are watermarking apps that might be able to batch add the same text to multiple photos as a time. Also GraphicConverter can do some batch editing and might be able to add the same text to multiple photos. You an use GC in demo mode to test it.

All of the above would have to be performed outside of the library so the photos would have to be exported to a folder first.

Do you mean to write text as a graphics overlay as a part of the photo, or do you want to add captions as apart of the meta data and displayed below the photo ?

If you want yo scribble text into ghe image, you can use the mark-up extension, to add graphics to the image in edit mode, but only for one image at a time. But you can batch replace the title of several selected images at once. the title can be viewed below the thumbnails or as a headline while viewing the photo enlarged.

FWIW I use GraphicConverter to add a Caption, Keywords, location, date, rating as a batch. Then import to Photos.

You can use the Markup feature in the Edit mode to add text to a photo:

It works just like in Preview.